Isocitrate dehydrogenase (ID) catalyzes the conversion of isocitrate to .alpha.-ketoglutarate, a substrate of the citric acid cycle. In doing so, ID also captures the reducing power of the reaction by transferring electrons from isocitrate to an electron acceptor. In this way, ID participates in generating ATP to power a cell's numerous energy-requiring reactions, and harnesses high energy electrons for subsequent oxidation/reduction reactions.
Eukaryotic organisms contain three isocitrate dehydrogenases (IDs), representing a well-conserved family of enzymes. Though there may be low overall sequence homology within this family of enzymes, there is a sequence of 20 amino acids near the C-terminal region called the isocitrate dehydrogenase signature sequence, which is highly conserved between family members. The three eukaryotic IDs fill different physiological roles, a requirement which is reflected in their localization in different cellular compartments, and in their interaction with distinct electron acceptors. Two of these IDs are associated with mitochondria but interact with different electron acceptors. One mitochondrial ID interacts specifically with the electron acceptor nicotinamide adenine dinucleotide (NAD), and is termed NAD-dependent, while the other mitochondrial ID interacts specifically with nicotinamide adenine dinucleotide phosphate (NADP), and is termed NADP-dependent. The third ID is termed cytosolic (cytosolic isocitrate dehydrogenase, CID), but may be found either in the cytosol or in peroxisomes. CID is NADP-dependent. Differences in functional roles of IDs relate to differences in electron acceptor requirements. NAD is the major electron acceptor involved in generation of ATP via oxidative phosphorylation in mitochondria. NADP is the major electron acceptor involved in biosynthetic pathways requiring reducing power, including the synthesis of cholesterol and bile acids, oxidation of D-amino acids, fatty acids and polyamines, and peroxide-based metabolism. Thus, ID couples the oxidative decarboxylation of isocitrate to the generation of molecules essential for the production of cellular energy (NAD) and for various biosynthetic reactions (NADP) (Jennings, G. T. et al. (1994) J Biol Chem 269: 23128-34).
The activity of cytosolic isocitrate dehydrogenase (CID) varies from tissue to tissue. In reproductive tissue such as the ovary and mammary gland, and in the liver, CID activity is high, while in heart and skeletal muscle CID activity is low. Liver tissue and reproductive tissues have high levels of NADP-dependent biosynthetic reactions. This tissue-specific difference in CID activity reflects the role of CID in generating NADP for biosynthetic reactions. In addition, CID activity is regulated during development. In the rat, CID activity is maximal in immature brain but decreases with age, while in the liver, where CID activity is highest of any tissue, the converse is true (Yadav, R. N. and Singh, S. N. (1980) Biochim Biophys Acta 633: 323-30). CID activity is also regulated by hormones. The activity of CID was dramatically increased in gonadotropin-induced development of the immature rat ovary. Similarly, the enzyme's activity is increased in the mammary gland at the onset of lactation. The regulation of CID activity by hormones and by cellular differentiation in reproductive tissues indicates a role for CID in cancer and disorders of the reproductive system (Jennings, G. T. and Stevenson, P. M. (1991) Eur J Biochem 198: 621-25).
Peroxisomes are membrane-delineated organelles found in nearly all eukaryotic cells. The interior, or matrix, of the peroxisome contains at least fifty enzymes, most of which are targeted to the peroxisome by a microbodies C-terminal targeting signal, typically SKL (ser-lys-leu). Peroxisomal enzymes catalyze a variety of metabolic reactions involved in reductive biosynthetic pathways, such as the synthesis of cholesterol and bile acids, the oxidation of D-amino acids, fatty acids and polyamines, and peroxide-based metabolism. These various metabolic processes require reducing power in the form of NADP. Thus, defects in the generation of NADP by CID in peroxisomes can result in disorders associated with peroxisome metabolism (Baumgart, E. et al. (1996) Proc Natl Acad Sci 93: 13748-53).
The discovery of a new human cytosolic isocitrate dehydrogenase and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention and treatment of cancer, reproductive disorders, and disorders of peroxisome metabolism.